1. Field of the Invention
The present invention relates to a receiver including a direct conversion circuit for use in a receiver which performs quadrature demodulation, and more particularly to a receiver which can improve an image signal rejection ratio and further reduce a load of hardware or processing.
2. Description of the Related Art
In a conversion circuit of a direct conversion system which is one of conversion systems for use in a receiver, and the like, when a modulated ratio wave is received, almost the same frequency of a carrier wave signal (local frequency) as that of a central frequency (desired reception frequency) of this received signal is output by a local oscillator, so that the received signal and a local oscillation signal from this local oscillator are mixed to thereby convert a reception wave in an RF band directly into a base band signal, which is then detected and demodulated.
By such a direct conversion system, however, since the frequency of a carrier wave output from the local oscillator is the same as a desired reception frequency, such a direct current (DC) offset phenomenon occurs that the output from the local oscillator is input again to another input of a multiplication process and multiplied again by the output of the local oscillator to thereby offset a DC component of the base band signal or such a fundamental problem occurs that 1/f noise is generated because an intermediate frequency of the base band signal obtained by the multiplication is nearly zero (0). This has made it difficult to receive signals stably in a broad band.
To solve the problem, there is a direct conversion circuit employing a low intermediate frequency (IF) system.
By a direct conversion method of the low IF system, an offset frequency (difference in frequency) having such an extent as not to generate a DC offset or 1/f noise is given between a desired reception frequency and a frequency output by the local oscillator, then converted to a low IF, so that the reception signal is converted in frequency by digital signal processing using the offset frequency. Accordingly, in-phase and quadrature outputs are obtained.
In the low IF system, however, it is necessary to suppress an image signal at an image frequency so that it may not overlap with the desired signal.
The above-described conventional direct conversion of the low IF system has a problem that it is difficult to sufficiently attenuate an image frequency signal contained in a reception signal with respect to a desired wave at a band pass filter. Therefore, it may sometimes be necessary to vary the intermediate frequency, band, and the like of the band pass filter.
That is, it is difficult to obtain wide band characteristics of a receiver including the direct conversion and so to implement a wide band low IF receiver.
To solve the above-mentioned problem of the image frequency signal, there is such a method as to give processing way for rejecting the image signal.
By this method, for example, in the low IF-type direct conversion receiver, a desired reception signal is multiplied by a local-oscillator's frequency provided with an offset frequency to be down-converted into an low-IF signal, which then undergoes image signal rejection and then digital signal processing to convert the offset frequency, thus obtaining in-phase and quadrature outputs.
Note here that one conventional technology related to a direct detection-system receiver is disclosed in Japanese Patent Application Laid-Open No. Hei 10-70482 published on Mar. 10, 1998 (Applicant: Philips Electronics., Nemrose, Fennohtsharp, Inventor: Paul, Anthony, Moore, et al.).
This conventional technology provides an integrated receiver using a local oscillator which oscillates an intermediate frequency of a desired signal wave higher than that of an interference signal wave directly detected to thereby down-convert the frequency of an input signal. Accordingly, the influence of an interference signal can be removed (see Patent Document 1).
Moreover, another conventional technology related to the direct conversion system is described in Japanese Patent Application Laid-Open No. 2001-77717 titled “Receiver” laid-open on Mar. 23, 2001 (Applicant: Toshiba Corp., Inventors: Hiroshi Tsurumi, et al.).
According to this conventional technology, there is provided a receiver for collectively receiving a system band which is an object to select a channel by digital processing. In the receiver, after the received signal is quadrature-demodulated at a local frequency, the image signal is suppressed. Alternatively, after the quadrature-demodulated signal is converted to a digital signal, the image signal is suppressed. A desired channel is demodulated by the digital processing, and selected. Accordingly, a broad band is collectively received, and flexible processing by the digital processing is possible. Moreover, sufficient image signal suppression is obtained (see Patent Document 2).
Furthermore, the conventional technology of a decimation filter related to the present invention is described in: Japanese Patent Application Laid-Open No. 5-175785 titled “Digital Filter for Decimation” laid-open on Jul. 13, 1993 (Applicant: Matsushita Electric Industrial CO., Ltd., Inventors: Tetsuhiko Kaneaki, et al.) (see Patent Document 3); Japanese Patent Application Laid-Open No. 5-299973 titled “Decimation Filter” laid-open on Nov. 12, 1993 (Applicant: Fujitsu Ltd., Inventors: Nobukazu KOIZUMI, et al.) (see Patent Document 4); Japanese Patent Application Laid-Open No. 10-209815 titled “Decimation Filter” laid-open on Aug. 7, 1998 (Applicant: Matsushita Electric Industrial CO., Ltd., Inventors: Hideaki Hatanaka, et al.) (see Patent Document 5); Japanese Patent Application Laid-Open No. 2001-77667 titled “Decimation Filter” laid-open on Mar. 23, 2001 (Applicant: Hitachi, Ltd., Inventors: Haruhiro Hasegawa, et al.) (see Patent Document 6); and the like.
Additionally, the conventional technology related to a receiver for demodulating a digital modulation signal using a digital signal processing technology is described in Japanese Patent Application Laid-Open No. 10-209904 titled “Receiver” laid-open on Apr. 7, 1998 (Applicant: Hitachi Electronics, Ltd., Inventors: Hirotake Wakai, et al.).
According to the conventional technology, there is provided a receiver including means for limiting a band of a frequency-converted signal to that of an IF signal by a band pass filter. After A/D conversion, deviation of the intermediate frequency is corrected by a complex processing circuit, and the signal is filtered in a narrow base band by the digital filter to obtain a demodulated output. Accordingly, band pass processing is carried out with a broad-band analog filter, and narrow-band filter processing is carried out after the A/D conversion, so that manufacturing cost of the analog filter is reduced. The circuit which obviates necessity for an IF filter equalizer for correcting characteristics can be realized (see Patent Document 7).
Patent Documents 1 to 7 denote Japanese Patent Application Laid-Open Nos. 10-70482, 2001-77717, 5-175785, 5-299973, 10-209815, 2001-77667, and 10-209904, respectively.
In the conventional direct conversion circuit, however, since image rejection processing is analog processing, coefficient errors of the analog filter are generated by fluctuations of an analog device, and there exists a problem that an image rejection ratio is not sufficient.
For example, when the fluctuation of a device value is 1% in terms of an average value, such a problem occurs that an image signal suppression ratio of about 40 dB is a limit in the analog processing and efficiency of image signal rejection is bad.